1. Field of the Invention
The present invention relates generally to a trailer having hydraulically powered driving axles and more particularly to controlling the driving torque of a vehicle, the trailer of which has hydraulically powered driving axles.
2. Discussion of Background
Although it is not limited to this particular application, the invention particularly applies to vehicles adapted to run on difficult terrain such as work sites, sandy ground and the like.
The problem which the invention aims to deal with is substantially to share the driving load between the tractor and its trailer, so as to get an all-purpose vehicle with driving wheels on the trailer as well as on the tractor (and preferably in which every wheel is a driving wheel), in order to avoid situations such as the vehicle getting stuck in sand, mud, and the like, the pulling of the entire load by the tractor due to too little thrust from the trailer, and jackknifing due to too much thrust from the trailer.
The invention particularly applies to a vehicle provided with a powering system known in the art where the motor of the tractor drives, through a gear box, on one hand its own driving wheels and on the other hand a pump feeding a hydraulic motor on the trailer, this latter motor then driving the driving wheels on the trailer.
Such a driving system, when appropriately designed particularly in regard to the sharing of the driving force between the tractor and the trailer, operates satisfactorily when the vehicle runs straight forward, and when the driving wheels each have the same traction, and thus run at the same speed.
However, in a curve, the tractor travels through a path with a larger radius than the path followed by the trailer and, without control device, the hydraulic motor, the speed of which is mechanically bound to the means speed of the driving wheels of the trailer, tends to slow down and to increase the pressure in the circuit.
On the other hand, in cross country use, the vehicle encounters varying traction conditions. Two conditions are indeed possible as a function of the loss of traction which occurs either on the tractor, or on the trailer. Without any correction applied to the system, when the tractor begins to slip, an increase of the output of the pump occurs. Since the speed of the pump is mechanically bound to the mean speed of the driving wheels of the tractor, the pressure in the hydraulic circuit is thus increased. So, the torque transmitted to the driving wheels of the trailer by the hydraulic motor also increases, since the hydraulic motor, the speed of which is mechanically bound to the mean speed of the driving wheels of the trailer, resists to this flow increase. If no correction is applied, this increase of the torque results in either the driving wheels of the trailer slipping when the traction is not good enough, or a chance that the vehicle could jackknife due to too much thrust from the trailer.
On the contrary, and for the same reasons, when the trailer starts to slip, a lowering of the pressure in the circuit occurs, and thus the driving torque from the trailer decreases. If no correction is applied, a major part, and possibly all the benefit of a powering of the trailer is lost.